1. Field of the Invention
The present invention relates to non-destructive testing of HEMT-like wafers, e.g., high electron mobility transistors (HEMT), pseudomorphic high electron mobility transistors (PHEMT), and double heterojunction high electron mobility transistors (DHPHEMT).
2. Description of Related Art and Other Considerations
HEMT-like wafers, as are manufactured, for example by a molecular beam epitaxy (MBE) method, are used as radio frequency and microwave frequency semiconductor devices. A typical wafer comprises a gallium arsenide substrate onto which deposits of five to ten layers, typically, are formed. The number of layers depends on the application to be used. These wafers are used to manufacture high electron mobility transistors (HEMT), pseudomorphic high electron mobility transistors (PHEMT), double heterojunction high electron mobility transistors (DHPHEMT), and the like.
Heretofore, there has been no effective way to non-destructively screen or test the electrical properties of HEMT-like or equivalently manufactured wafers for quality. The only method, to determine whether a wafer was satisfactory, was to build a device and then destructively test the device, such as by Hall measurements. This method is slow, time consuming, expensive and, because the tested wafer is destroyed, precludes 100% testing and reuse of the tested wafer.
An additional problem with conventional methods is that, in HEMT-like wafers having four to five or more layers, it is very difficult or even impossible to screen them for the properties of the critical layers. There is no way to distinguish whether the critical layers are good or bad. Thus, it has been necessary to utilize the expensive technique of constructing a wafer, at a cost from $5,000 to $10,000 per wafer, to determine if the wafer was good or bad.
Furthermore, if the surface of the wafer under test is irregular, such as having bad surface morphology with small textures, it is not possible to utilize conventional testing techniques.